Does anyone know how to successfully pack linked objects (not just textures) into a single .blend file?
To quickly get all the linked data in one .blend file you can use Make Local command.




Select all the objects which are linked (or simply A to select all).
Press L and select All option. This will make all the objects, their data, materials etc. stored locally in this file, so create a full copy (in the same way as Append data, with only difference that you shouldn't search where to append from). 


Note that you may want to save these locally stored objects as a new file so only to send it to renderfarm and not ruin linking of your original file.